The present invention relates to a plasma display device; and, more particularly, to a plasma display device including plasma display panel as a light emitting device using an Eu activated silicate phosphor which emits light when excited by ultraviolet light in the vacuum ultraviolet region.
In recent years, there has been a high demand for thin type display devices represented by TV sets or PC monitors to save installation space, without having to secure large space for them. Some of eligible examples of slim display devices that have drawn a lot of attention are a plasma display device (hereinafter, PDP) designed to display images using a plasma display panel with a driving unit for the PDP, which is thinner than the cathode ray tube as the light emitting device, a liquid crystal display device designed to display images using a fluorescent lamp as the light emitting device in combination with a liquid crystal display panel, and so forth.
The PDP configuring a plasma display device emits light in the visible region by exciting a phosphor in the phosphor layer upon irradiation with ultraviolet light emitted in a negative glow region in the micro discharge space as an excitation source and thereby accelerating light emission of the phosphor (central wavelengths of the main emission of the ultraviolet light are between 146 nm and 172 nm when xenon gas is used as the rare gas). The plasma display deices display images by controlling the intensity and color of the light emission of the plasma display panel.
Nowadays, there is an increasing demand for plasma display devices with higher luminance and higher emission efficiency to satisfy the display function, particularly for TVs, and so on. Plasma display devices are also required to have better video characteristics for allowing viewers to comfortably appreciate the video contents such as movies, and to ensure a wide color reproduction range of 100% or more with respect to the NTSC (national television system committee) for allowing viewers to enjoy beautiful images. Further, plasma display devices are required to have improved reliability for enabling viewers to have the good image quality over a long period of time.
In order to make progress in plasma display devices of high performance in satisfaction of such needs, it is pretty much crucial to achieve higher performance of PDP that constitutes a plasma display device. More specifically, PDPs should keep abreast with demands in the high luminance for the display function for the TV, the high emission efficiency for achieving the high luminance, improvement of response performance for coping with improvement of video quality, enlargement of color reproduction range, and further, improvement of reliability.
For improvement of performance and characteristics of the PDP, improvement of design and structure and improvement of performances of the members of the PDP play significant roles. Improvement of color reproduction performance and improvement of reliability depend largely on the phosphor among the constituent members in particular. Therefore, the phosphor is highly required to demonstrate higher emission efficiency, better response characteristics in the light emission, and secured color reproduction properties. In addition, performance improvement of deterioration resistance and further, improvement of reliability are highly required.
As phosphors of the current color PDP, phosphors corresponding to three emission colors of red (R), blue (B) and green (G), in other words, a red-emitting phosphor, a blue-emitting phosphor and a green-emitting phosphor are used. As the blue-emitting phosphor, an Eu activated aluminate phosphor (BaMgAl10O17:Eu, hereafter, it is referred to as BAM) is broadly used. BAM has excellent characteristics in its light emission but is susceptible to deterioration, in other words, BAM has poor reliability and short life. Thus, it is demanded to make improvement in stability, to achieve long life, and to realize higher photoluminescent brightness. Meanwhile, as Digital Hi-Vision equipment is going to come in wide use in near future, it is also necessary to cover chromaticity scale of the HDTV standard, the standard for Hi-Vision. To comply with the standard, blue chromaticity y value (0.06) should be smaller than NTST chromaticity y value (0.08). In effect, a smaller y value, i.e., deep-blue color, is required to meet the need for blue-emitting phosphor. If it is possible to get even a deeper blue color, plasma display devices are going to have a broader color reproduction range, expressing more varieties of colors.
Moreover, in the technical field of the PDP in parallel with the discussion for achieving high performance of the phosphor materials, many manufacturers have been trying to improve panel structure aiming at high emission efficiency of PDPs.
As a specific method thereof, a technique for actively utilizing Xe2 molecular line (wavelength 172 nm) generated by the electric discharge by increasing the mole fraction of xenon contained in a discharge gas has actively been discussed. Such orientation of the technique is referred to as the trend of the so-called “high Xenon-content” technology in PDP. The main focus of the “high Xenon-content” technology being studied is to increase the mole fraction of Xe in the discharge gas up to 4% or higher, so that high efficacy PDP panels may be achieved.
Under these circumstances, a silicate phosphor is proposed as a blue-emitting phosphor having higher reliability and longer life than BAM of conventional blue-emitting phosphor. To be more specific, Ca1-xMgSi2O6:Eux (hereafter, it is referred to as CMS) (refer to Japanese Patent Application Laid-Open Publication No. 2002-332481), Sr3-xMgSi2O8:Eux (hereafter, it is referred to as SMS) (refer to Japanese Patent Application Laid-Open Publication No. 2003-336048), and (BaxSr1-x)3-yMgSi2O8:Euy (hereinafter, it is referred to as BSMS) (refer to Japanese Patent Application Laid-Open Publication No. 2006-12770) formed by substituting part of the host lattice with Ba on the basis of SMS were proposed.
The CMS mentioned above emits light having relatively high brightness and good color purity upon, given ultraviolet light in a wavelength range of 146 nm as the excitation source. However, it is known that the excitation band barely exists in a wavelength range of 160 nm to 210 nm as its excitation characteristics. Therefore, the intensity of emission to be generated by the excitation by vacuum ultraviolet light at the vicinity of 172 nm (Xe2 molecular line), which is important for the PDP, is extremely low. In other words, from the aspect of the technical trend heading for high efficacy PDPs, CMS has a low emission efficiency with respect to the Xe2 molecular line at an increasing wavelength of 172 nm, so the effects of substantial improvement in luminance and efficiency are not obtained. Accordingly, when it comes to CMS, apart from its insufficient brightness as remarked under given circumstances and the current technical trend heading for high efficacy PDPs, the CMS is required to have higher emission efficiency in the excitation band of 172 nm wavelength, making practical improvement in real life. Particularly, the silicate phosphors SMS and BSMS disclosed in Japanese Patent Application Laid-Open Publication Nos. 2003-336048 and 2006-12770 are promising novel phosphors because they have high brightness characteristics upon an excitation by light at a wavelength of 146 nm and shows a good emission efficiency also upon an excitation by light at a 172 nm wavelength as compared to CMS.
Meanwhile, part of the fabrication of PDP involves applying a phosphor paste containing phosphor and organic substances as host material onto a substrate, and heating the substrate at a temperature range of 300° C. to 600° C. to obtain phosphor layers. However, it turned out the SMS-based phosphor paste did not much contribute to brightness and chromaticity of resulting phosphor layers probably because it has gone though the heating process. Japanese Patent Application Laid-Open Publication No. 2003-336048 suggests one way of preventing process degradation and operating degradation by heating the prepared SMS phosphor under the oxidizing atmosphere to intentionally increase the substitution ratio of Eu3+ ions, thereby decreasing the oxygen defects present in a phosphor crystal.